Late potentials are considered to be a marker for regional slow conduction which might predispose to reentrant ventricular arrhythmias. Since these arrhythmias may be induced by ischaemia it may be speculated that exercise-induced myocardial ischaemia may trigger late potentials. Exercise testing was performed in 53 patients early after myocardial infarction and in 20 healthy controls. Typical 12 lead ECG and signal averaged ECG (SA-ECG) from 12 leads were recorded before and after exercise testing. Changes in filtered QRS (QRS) and low amplitude signal durations, and in the root mean square voltage of the last 40 ms of the QRS complex (RMS40) were analysed. Thirty patients developed ST changes, consistent with transient ischaemia, that persisted during the SA-ECG recording after exercise. There were significant differences between baseline SA-ECG and SA-ECG after exercise in patients with positive exercise tests (QRS, 102 +/- 15 ms vs 114 +/- 15 ms (P < 0.01). LAS, 36 +/- 12 ms vs 42 +/- 11 ms (P < 0.05), and RMS40, 29 +/- 14 microV vs 20 +/- 13 microV (P < 0.01). No differences were observed in SA-ECG parameters in either patients with negative exercise tests or in controls. During follow-up, four patients died suddenly; all four had positive exercise tests and in three of them late potentials were induced by exercise. We conclude, that in post-infarction patients with positive exercise tests SA-ECG parameters deteriorate after exercise. This suggests that exercise-induced ischaemia triggers development of late potentials.